libexec: detect short files

if an exec() fails partway through reading in the sections, the target
process is already gone and a defunct process remains. sanity checking
the binary beforehand helps that.

test10 mutilates binaries and exec()s them on purpose; making an exec()
fail cleanly in such cases seems like acceptable behaviour.

fixes test10 on ARM.

Change-Id: I1ed9bb200ce469d4d349073cadccad5503b2fcb0

kernel/arch/earm/protect.c

@@ -131,7 +131,7 @@ void arch_boot_proc(struct boot_image *ip, struct proc *rp)
 		execi.stack_size = 32 * 1024;	/* not too crazy as it must be preallocated */
 		execi.proc_e = ip->endpoint;
 		execi.hdr = (char *) mod->mod_start; /* phys mem direct */
-		execi.hdr_len = mod->mod_end - mod->mod_start;
+		execi.filesize = execi.hdr_len = mod->mod_end - mod->mod_start;
 		strcpy(execi.progname, ip->proc_name);
 		execi.frame_len = 0;
 

kernel/arch/i386/protect.c

@@ -407,7 +407,7 @@ void arch_boot_proc(struct boot_image *ip, struct proc *rp)
 		execi.stack_size = 16 * 1024;	/* not too crazy as it must be preallocated */
 		execi.proc_e = ip->endpoint;
 		execi.hdr = (char *) mod->mod_start; /* phys mem direct */
-		execi.hdr_len = mod->mod_end - mod->mod_start;
+		execi.filesize = execi.hdr_len = mod->mod_end - mod->mod_start;
 		strlcpy(execi.progname, ip->proc_name, sizeof(execi.progname));
 		execi.frame_len = 0;
 

lib/libexec/exec_elf.c

@@ -156,6 +156,15 @@ int libexec_load_elf(struct exec_info *execi)
 	assert(execi->allocmem_prealloc);
 	assert(execi->allocmem_ondemand);
 
+	for (i = 0; i < hdr->e_phnum; i++) {
+		Elf_Phdr *ph = &phdr[i];
+		off_t file_limit = ph->p_offset + ph->p_filesz;
+		/* sanity check binary before wiping out the target process */
+		if(execi->filesize < file_limit) {
+			return ENOEXEC;
+		}
+	}
+
 	if(execi->clearproc) execi->clearproc(execi);
 
 	for (i = 0; i < hdr->e_phnum; i++) {

lib/libexec/libexec.h

@@ -28,6 +28,7 @@ struct exec_info {
     int allow_setuid;                   /* Allow set{u,g}id execution? */
     vir_bytes stack_size;		/* Desired stack size */
     vir_bytes load_offset;		/* Desired load offset */
+    off_t filesize;			/* How big is the file */
 
     /* Callback pointers for use by libexec */
     libexec_loadfunc_t copymem;		/* Copy callback */

servers/rs/exec.c

@@ -124,7 +124,7 @@ static int do_exec(int proc_e, char *exec, size_t exec_len, char *progname,
 	execi.stack_size = DEFAULT_STACK_LIMIT;
 	execi.proc_e = proc_e;
 	execi.hdr = exec;
-	execi.hdr_len = exec_len;
+	execi.filesize = execi.hdr_len = exec_len;
 	strncpy(execi.progname, progname, PROC_NAME_LEN-1);
 	execi.progname[PROC_NAME_LEN-1] = '\0';
 	execi.frame_len = frame_len;

servers/vfs/exec.c

@@ -312,6 +312,7 @@ int pm_exec(endpoint_t proc_e, vir_bytes path, size_t path_len,
 
   execi.args.proc_e = proc_e;
   execi.args.frame_len = frame_len;
+  execi.args.filesize = execi.vp->v_size;
 
   for (i = 0; exec_loaders[i].load_object != NULL; i++) {
       r = (*exec_loaders[i].load_object)(&execi.args);

servers/vm/main.c

@@ -276,6 +276,7 @@ void exec_bootproc(struct vmproc *vmp, struct boot_image *ip)
         strlcpy(execi->progname, ip->proc_name, sizeof(execi->progname));
         execi->frame_len = 0;
 	execi->opaque = &vmexeci;
+	execi->filesize = ip->len;
 
 	vmexeci.ip = ip;
 	vmexeci.vmp = vmp;

test/test10.c

@@ -77,7 +77,11 @@ int n;
   if ((pid = fork()) != 0) {
 	wait(&n);		/* wait for some child (any one) */
   } else {
-	execl(name[n], name[n], (char *) 0);
+	/* a successful exec or a successful detection of a broken executable
+	 * is ok
+	 */
+	if(execl(name[n], name[n], (char *) 0) < 0 && errno == ENOEXEC)
+		exit(0);
 	errct++;
 	printf("Child execl didn't take. file=%s errno=%d\n", name[n], errno);
 	rmfiles();